Osteoporosis, a severe aging disease of the skeleton, gains ever growing importance due to the continuously increasing life expectancy. The annual cost to the health care system due to osteoporosis is estimated to amount up to several billions worldwide, in Germany one estimate is about 1 billion. Clinical and measurement efforts for an early diagnosis and efficient control of therapy are accordingly high.
Two procedures have shown to be particularly suitable for bone mineral measurements:
Photon absorptiometry (DXA, Dual Energy X-ray Absorptiometry) and quantitative computed tomography (QCT). For each modality there are more than 1000 installations of various manufacturers in use worldwide.
A particular problem and serious controversies have evolved from the fact that results which have been obtained on different units do not agree or can hardly be compared, respectively. In particular, for the DXA scanners, each manufacturer uses different phantoms both for calibration and for quality control. Due to the fact that different materials and different geometries are employed, calibrations are different Differences result for measurements of the same object or patient on different apparati. The phantoms available mostly offer homogeneous bone without separation of spongious and cortical portions. Due to their anthropomorphic shape, it is not possible to determine if the presented area values and the directly resulting area density values are correct.
In QCT, the manufacturers do not offer anthropomorphic phantoms. There are only some smaller specialized companies which offer such phantoms. These are not useful for direct comparison to DXA; also, they do not offer defined cortical structures. Since so far only solutions specific to one manufacturer and one scanner type have been offered, controversies resulted, but no accepted solution to the problem of cross-calibration of different devices. In addition, it has to be stated that the phantoms offered by small specialized companies are very expensive, partly not very practical and they do not offer all the desired test procedures.
It appears desirable to have only one phantom to simulate bone mineral measurements of the lumbar spine with both absorptiometry and QCT. It will help to reduce cost to have only one phantom instead of two different ones in those institutions who use both modalities. More important, however, is the intent to avoid a diversity of different phantom geometries and materials. This would be counterproductive in any standardization effort, and it might reinforce confusion or irritations in the user community.
For DXA, both a.p. and lateral measurement capabilities have to be provided. For QCT, separate measurement of spongious and cortical bone is required. The phantom should allow testing of reproductibility and accuracy of machines, both in clinical installations and at manufacturers' sites, in determining the following quantities:
projected area of vertebrae in cm.sup.2 for DXA, PA1 bone mineral content (BMC) in g for DXA, PA1 trabecular and cortical bone mineral density in g/cm.sup.3 for QCT, PA1 cortical thickness in mm for QCT, PA1 positioning in QCT. PA1 for all of the above measurements, the true values have to be known and defined in an objective manner; PA1 the phantom has to be designed in a way such that results of phantom measurements model the situation of patient measurements appropriately.
Bone mineral area density (BMD) in g/cm.sup.2, which is the quantity most often quoted in DXA, results directly from the above. Tests of linearity have to be provided also and require that the phantom offers either multiple sections or multiple inserts. The respective sections of inserts should cover the range of density values typically encountered in patients.
To perform appropriately, two general demands on the phantom must be met:
The first demand is absolutely necessary for the phantom to be used as a standard. The second demand is a logical one if the phantom is to give relevant results; in a particular, this means that cross-calibration factors obtained on the phantom have to be transferable to patient measurements.